Secondary cracking along the grain boundaries near the fracture surface has been observed in various materials including brittle ceramics, polymers, and metals. The mechanism of secondary cracking is largely attributed to the formation and propagation of microcracks initiated at the grain boundaries due to internal stress concentrations. This causes the fracture surface to appear roughened as a result of intergranular embrittlement.
The observation of secondary cracking has been key in the understanding of the fracture behavior of materials, since it provides valuable information about the role of microstructure on the initiation, propagation, and acceleration of fatigue and fracture. The presence and extent of secondary cracking is evaluated visually, through macroscopic examination. In metals, this is characterized by the presence of transverse or cross-grained fractures that propagate along the grain boundaries.
In brittle ceramics, such as alumina, secondary cracking is typically characterized by the presence of finite-sized vertical lines, termed “shards”, at random angles and lengths along the fracture surface, where transverse cracking has been observed in metals . Furthermore, blunting of the grain boundaries, or even the disappearance of a distinct boundary, is sometimes observed.
In addition to the visual examination, secondary cracking can also be evaluated by further microscopic methods. In literature, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are the most common methods used in assessing the extent and degree of secondary cracking. However, in cases where physical contact is not desirable or possible, eddy current (EC) testing has also been suggested as a viable non-contact alternative.
In this study, an EC probe was used to assess the secondary cracking along grain boundaries near the fracture surface of a red-colored alumina samples. A commercial EC probe with a cylindrical eddy-current sensor head was employed for the measurements. The EC head was positioned close to the fracture surface of the samples and oscillated over a specified area. As the EC probe was passed over the area of interest, the sample was monitored for changes in the measured signal. When a significant change in the signal amplitude was observed, this indicated the presence of secondary cracking along the grain boundaries near the fracture surface. The areas where secondary cracking was observed were marked for further characterization with conventional microscopic microscopy.
The results of this study indicate that EC testing can be successfully applied to identify the presence and extent of secondary cracking along the grain boundaries near the fracture surface of alumina samples. The results imply that EC testing is a viable alternative to conventional microscopy methods, especially in cases where physical contact with the samples is not desirable. Furthermore, the results of this study provide valuable insight into the role of microstructure on the initiation and propagation of fatigue and fracture in materials.
